A thread monitor, known from U.S. Pat. No. 4,170,951, is arranged on a sewing machine in the path of the needle thread and is provided with a transducer with a spring clip, to which a wire strain gauge element, hereinafter called a WSG element, is fastened. The WSG element produces an electrical voltage that is proportional to its mechanical deformation caused by the deflecting movement of the spring clip. The electrical voltage is fed into an evaluating electronic unit following the transducer.
In the course of a stitch, a first, lower tension level is produced during the expansion of the needle thread loop, and a second, higher tension level is generated during the knotting. To detect missed stitches, the tension levels are monitored in measuring windows, whose positioning and size are predetermined by two signal generators that monitor the position of the arm shaft.
A plurality of actual values are determined from the lower tension level, and compared with a threshold value, whose value depends on the maximum of the higher tension level generated during the preceding stitch. If all actual values are below this threshold value, a warning signal is sent by the evaluating electronic unit to indicate a missed stitch.
Since the maximum of the higher tension level may differ from the corresponding value of the preceding stitch during each stitch, and the threshold value depends on this maximum, the threshold value is to be determined anew for each stitch. Such a signal evaluation is problematic especially at high sewing speeds and requires the use of an expensive evaluating electronic unit.
Due to its dependence on the preceding higher tension level, the threshold value cannot be formed during the first stitch performed with ordinary tension after start-up of the sewing machine, which causes a delay in the monitoring process. To prevent this disadvantage, additional control elements not disclosed in U.S. Pat. No. 4,170,951 are necessary. This increases the circuit complexity of the evaluating electronic unit, which is suitable exclusively for indicating missed stitches caused by lack of expansion of the needle thread loop.